SolidWorks Simulation reporting to the roof for snow duty

This past weekend was all about snow and ice, or rather the removal of those two pesky forms of water. The last two winter storms have dropped a significant amount of snow here in the Boston area. Now, I am new to the area, hailing from England, and to be honest I didn’t pay much attention to it until the radio started reporting roof collapses and my neighbors got up on their roofs and started shoveling snow. So I did the same–I shoveled snow and removed ice. Although this was my first time doing this I had a sense of déjà vu. When had I done this before?

While taking a well earned rest (and having a nice cup of tea) it hit me. Ten years ago, during my first simulation class using COSMOS (which became SolidWorks Simulation), I had done an analysis of snow on a corrugated iron roof! With the roof cleared it was time to dig out my old notes. Being an engineer pack rat nothing is thrown away, and I was curious to see how much weight I had shifted off my roof.

According to my notes, snow is roughly 1/10th the density of water, at 100 kg per cubic meter (or 6.242 lbs per cubic ft). I cleared a good third of meter of snow off my roof, and with an area of about 150 square meters, that was a whooping 5,000 kg ( 5 .5 tons)!!!

No wonder the pretty white stuff can bring down a building.

Now I'm not an architect, but I wanted to see if SolidWorks could help me understand what’s going on. With SolidWorks CAD I can quickly rough up a rough roof truss–about 400 sq meters.

After jumping across to SolidWorks Simulation, I can hold down the the roof where it would attach to the frame, apply gravity and my snow load, and see what happens.

Looking at the stress result I can see that I have a safety factor of 4, which is a ratio of the maximum stress in the roof to the materials strength. But to me that seems a bit low. To improve the safety factor, I'll add more roof trusses.

Then I recalculate my answer.

My factor of safety is now 6, which makes me feel better, but looking at the roof deflections, I see that some of the trusses are deflecting over 100mm (4 inches). That doesn't exactly inspire confidence in a roof.

This looks better.

So one last change–two trusses, a couple of longitudinal braces and a change of wood.